I think Vilage stated earlier on the "other" interest check that floundered that in the military history of Rome, Roman focus shifted based on the enemies it was fighting. So it could be reasonable to assume that over a more prolonged survival the Empire of Rome chould likely phase certain aspects in and out. And Rome currently is likely used to dealing with more heavily-armored forces such as the equally immobile Sassanids and/or even the Germanic tribes (although I don't know much about Frankish and Germanic warfare tbh). So their army would be structured to deal with those threats in particular. Any restructuring too could imply that they'd leave themselves vulnerable to any one of their own enemies if they chose to adapt to lighter-forces.

It might also be argued that the Rashidun can more readily shift than Rome or Persia since they are young. All they'd need is the economic capability to do so. Muslim armies at this time were largely all infantry (it wasn't until the Levant and Mesopotamia were in Rashidun hands that the army could shift from 20% cavalry to 50-50). The Rashidun too could also count on using groups native in the Empire, but those who don't necessarily hinge to Rome being potentially loyal assets. Groups native to Arabia even - the Taghlib mostly - were Christian but didn't hinge on the whims of Byzantium, and presumably they wouldn't be loyal to Rome even if they're still Christian so could still be an indirect asset to the Rashidun or be an immense group to move into occupied territory to bend the demographics to Rashidun favor and shore up Latinized regions with simply more Bedouins.

General Arab activity too even in their conquest could as well contribute to them shoring up well, even if they don't go beyond the Levant. They may not have absolute loyalty, but they'd recognize the necessity of local populations to aid in the administration and we can propose that they could Romanize themselves pretty fast, provided they get the access to resources.

And then there is of course the ever lingering specter of some pissy general thinking he can do the Emperor one better and demanding the title through force of arms. The lineage of Roman Emperors isn't much of a unbroken line of dynastic rule.


But all-in-all this might lead into the world of excess OOC banter which can be poisonous. I don't like to see these implications and such explored in OOC but would rather the creative and mental effort but towards writing IC posts. If we hack it all out here outside the story then we'll have no energy or wonder yet to write it ourselves.